Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

Understanding Neutron Radiography 

Reading VIII Part 2(a) of 2 

16 th August 2016 

Post Exam Reading 

Charlie Chong/ Fion Zhang

Spallation Source 





Charlie Chong/ Fion Zhang 

http://www.gizmodo.com.au/2014/01/27-amazing-images-from-the-depths-of-scientific-labs/



http://www.gizmodo.com.au/2014/01/27-amazing-images-from-the-depths-of-scientific-labs/

The Magical Book of Neutron Radiography 



ASNT Certification Guide 

NDT Level III / PdM Level III 

NR - Neutron Radiographic Testing 

Length: 4 hours Questions: 135 

1. Principles/Theory 

• Nature of penetrating radiation 

• Interaction between penetrating radiation and matter 

• Neutron radiography imaging 

• Radiometry 

2. Equipment/Materials 

• Sources of neutrons 

• Radiation detectors 

• Non-imaging devices 


3. Techniques/Calibrations 

• Blocking and filtering 

• Multifilm technique 

• Enlargement and projection 

• Stereoradiography 

• Triangulation methods 

• Autoradiography 

• Flash Radiography 

• In-motion radiography 

• Fluoroscopy 

• Electron emission radiography 

• Micro-radiography 

• Laminography (tomography) 

• Control of diffraction effects 

• Panoramic exposures 

•Gaging 

• Real time imaging 

• Image analysis techniques 


4. Interpretation/Evaluation 

• Image-object relationships 

• Material considerations 

• Codes, standards, and specifications 

5. Procedures 

• Imaging considerations 

• Film processing 

• Viewing of radiographs 

• Judging radiographic quality 

6. Safety and Health 

• Exposure hazards 

• Methods of controlling radiation exposure 

• Operation and emergency procedures 

Reference Catalog Number 

NDT Handbook, Third Edition: Volume 4, 

Radiographic Testing 144 

ASM Handbook Vol. 17, NDE and QC 105 



Fion Zhang at Copenhagen Harbor 

16 th August 2016


SME- Subject Matter Expert 

http://cn.bing.com/videos/search?q=Walter+Lewin&FORM=HDRSC3 

https://www.youtube.com/channel/UCiEHVhv0SBMpP75JbzJShqw

Gamma- Radiography 

TABLE 1. Characteristics of three isotope sources commonly used for 

radiography. 

Source 

T½ 

Energy 

HVL 

HVL 

Specific 

Dose rate* 

Pb 

Fe 

Activity 

Co60 

5.3 year 

1.17, 1.33 MeV 

12.5mm 

22.1mm 

50 Cig -1 

1.37011 

Cs137 

30 years 

0.66 MeV 

6.4mm 

17.2mm 

25 Cig -1 

0.38184 

Ir192 

75 days 

0.14 ~ 1.2 MeV 

4.8mm 

? 

350 Cig -1 

0.59163 

(Aver. 0.34 MeV) 

Th232 

0.068376 

Dose rate* Rem/hr at one meter per curie 


八千里路云和月 



闭门练功 



http://greekhouseoffonts.com/


Whole Chapter 5 

Radiation Measurement 


PART 1. Principles of Radiation Measurement 

Emissions from radioactive nuclei and radiation from that portion of the 

electromagnetic spectrum beyond the ultraviolet energies can cause the 

ionization of atoms and molecules. Ionizing radiation occurs as three forms: 

(1) charged particles such as alpha particles, beta particles and protons, 

(2) uncharged particles such as neutrons and 

(3) electromagnetic radiation in the form of X-rays and gamma rays. 


Radiation Detection Systems 

Some forms of radiation, such as light and heat, can be detected by human 

sense organs; ionizing radiation, however, can be detected only by the after 

effect of itsionizing properties. If ionizing radiation does not interact with 

matter, its detection and measurement is impossible. For this reason, the 

detection process uses substances that respond to radiation, as part of a 

system for measuring the extent of that response. The ionization process 

isused by a large class of detection systems, including: 

■ ion chambers, 

■ proportional chambers, 

■ geiger-müller counters and 

■ semiconductor devices (Table 1). 

■ Some systems depend on the excitation and molecular dissociation 

( 分子离解 ) that occur with ionization. These processes are useful in (1) 

scintillation counters and (2) chemical dosimeters. Although other types of 

detection systems exist, they are not generally used in radiation survey 

instruments. 


TABLE 1. Effect of detected and measured ionization. 


PART 2. Ion Chambers and Proportional 

Counters 

Principles of Ionization 

The mechanism most widely used in radiation survey applications is the 

ionization principle: charged particles producing ion pairs by direct interaction. 

These charged particles may (1) collide with electrons and remove them from 

their atoms or (2) transfer energy to an electron by the interaction of their 

electric fields (Fig. 1). If the energy transfer is not sufficient to completely 

remove an electron, the atom is left in a disturbed or excited state. 

Gamma and X-ray photons interact with matter mainly by: 

■ photoelectric absorption, 

■ compton scattering and 

■ pair production, 

each of which produces electrons and ions that may be collected and 

measured. 


The average energy expended in the creation of an ion pair, in air and most 

gases, is about 34 eV) . 

The number of ion pairs produced per unit of path length is called specific 

ionization. Specific ionization is affected by the energy of the particle or 

photon by its change and by the nature of the ionized substance. 


FIGURE 1. Ion pair (showing ejected electron and vacancy in electron orbit of 

atom). 

pair 

34 eV for Air 


Ionization Chambers 

In an ionization chamber, an electric field is applied across a volume of gas, 

between two electrodes. Often the chamber’s geometry is cylindrical, a 

cylindrical cathode enclosing the gas and an axial, insulated rod anode 

(Fig. 2). Charged particles, photons or both pass through the chamber and 

ionize the enclosed gas. When an electric field is applied to the gas, ions drift 

along the electrical lines of force to produce an ionization current. Under 

normal conditions, electrons drift at speeds of about 104 m·s –1 (22 000 mi·h –1). 

The drift velocity of positive ions is many orders of magnitude less. When the 

electric field is increased slightly from zero and a detector is placed in the 

constant radiation field the collected ions still will be few in number because 

many recombine. As the voltage is further increased, recombination yields to 

ionization, where all ions are collected (Fig. 3). 


FIGURE 2. Basic ionization chamber with high value resistance R and 

voltage V. 


Electromagnetic Energy Interactions 

Photons interact with subatomic structures in one of the following three ways: 

•Photoelectric absorption 

•Compton Scatter 

•Pair production 

The particular type of interaction reflects probability statistics based on both 

the energy of the photon and the atomic number of the traversed atom. For 

most tissues of the body, average atomic number does not vary greatly – 

though cortical bone has the highest effective atomic number. 


https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-02.html

Photoelectric Absorption 

An atom completely absorbs a photon, which then disappears; this excess 

energy provided to the atom ultimately results in ejection of an orbital electron. 

The ejected electron is known as a photo electron. 

Electrons have binding energies of orbit, with outer shells having less energy 

than those closer to the positive nucleus. When the initial orbital vacated is 

not that of an outer valance electron, the atom remains in a high-energy state 

until an outer orbital electron shifts to fill its incomplete inner shell. This shift is 

accompanied by emission of a characteristic X-ray. 

Photoelectric Absorption is an important interaction for low energy photons 

(

True or False? 

The "photo electron" and free radical can interact with other molecules -- 

ultimately leading to ionizations and bond breakage, which are the 

biologically-important molecular manifestations of radiation damage. 



Compton Scatter 

Collision of a photon with an electron increases the kinetic energy of the 

electron. Thus set in motion, the electron is known as a recoil electron. The 

incident photon’s energy is not necessarily depleted, but it will diverge from its 

path and have lower energy -- i.e., it has a longer wavelength after collision. 

In diagnostic radiology, such scattered photons may lower contrast and thus 

degrade quality of the radiographic image. 

Important interaction for intermediate-energy photons (100 KeV to 10 MeV) 

Note: 

0.34 ~ 1.2 MeV (Exam Q) 



Compton Scatter 



Pair Production 

Note pair production requires relatively high photon energies that are 

generally not produced in diagnostic imaging. 

Photons with quantum energy in excess of 1.02 MeV (usually >10 MeV) may 

interact with matter to produce a negative electron and its anti-particle 

(positron). The value of 1.02 MeV equals the combination rest mass energy of 

an electron and a positron. 




0.51MeV 

0.51MeV 



Linear Energy Transfer [LET] 

LET is the amount of energy transferred to the local environment in the form 

of ionizations and excitations. Thus, LET indicates the potential for 

biologically important damage from radiation. 

Linear Energy Transfer can be thought of in two ways: 

• an average energy for a given path length traveled or 

• an average path length for a given deposited energy. 

The standard unit of measure is keV/um. 



Ionization tracts. When particulate or electromagnetic energy penetrates a 

cell, one or more ionizations will likely take place. While the precise site of 

interaction is somewhat random, ionizations will distribute along distinct paths. 

The density of ionizations along a given path relates inversely to kinetic 

energy of the particle or photon. 

Thus a decelerating particle produces the greatest number of ionizations just 

before coming to rest. Comparing particles or photons, it follows also that LET 

for a gamma ray may be smaller than LET for an x-ray. 



Simulation of various radiation energies passing through a medium – each 

hatch mark represents an ionization. The heavy ion is a very high-LET 

particle; the delta ray represents secondary electrons with sufficient energy to 

make a separate ionization tract. The 5 keV electron is the typical energy of a 

secondary electron produced by X-ray photons used in diagnostic imaging. 

Note that absorption (and attenuation) of a photon beam is related to the 

atomic number of the impinged mass and inversely related to the energy of 

the incident photon beam. 

The medium shown is approximately 200 nm in width – a DNA double helix 

width is about 2 nm. 

(Developed after Cox J.D. and Ang K.K., eds. Radiation Oncology Rationale, 

Technique, Results. 8th edition. St Louis, MO: Mosby, 2003. p44.) 



Simulation of various radiation energies passing through a medium 



Sources of Attenuation 

The attenuation that results due to the interaction between penetrating 

radiation and matter is not a simple process. A single interaction event 

between a primary x-ray photon and a particle of matter does not usually 

result in the photon changing to some other form of energy and effectively 

disappearing. Several interaction events are usually involved and the total 

attenuation is the sum of the attenuation due to different types of interactions. 

These interactions include the photoelectric effect, scattering, and pair 

production. The figure below shows an approximation of the total absorption 

coefficient, (µ), in red, for iron plotted as a function of radiation energy. 

The four radiation-matter interactions that contribute to the total absorption 

are shown in black. The four types of interactions are: photoelectric (PE), 

Compton scattering (C), pair production (PP), and Thomson or Rayleigh 

scattering (R). Since most industrial radiography is done in the 0.1 to 1.5 MeV 

range, it can be seen from the plot that photoelectric and Compton scattering 

account for the majority of attenuation encountered. 


https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm

Total Absorption Coefficient, (µ), 

µ 

photoelectric (PE), 

Compton scattering (C), 

pair production (PP), and 

Thomson or Rayleigh scattering (R). 



Coherent scattering (also known as unmodified, 

classical or elastic scattering) is one of three forms of photon 

interaction which occurs when the energy of the X-ray or gamma photon is 

small in relation to the ionisation energy of the atom. It therefore occurs with 

low energy radiation. 

Upon interacting with the attenuating medium, the photon does not have 

enough energy to liberate the electron from its bound state (i.e. the photon 

energy is well below the binding energy of the electron) so no energy transfer 

occurs. The only change is a change of direction (scatter) of the photon, 

hence 'unmodified' scatter. Coherent scattering is not a major interaction 

process encountered in radiography at the energies normally used. 

Coherent scattering varies with the atomic number of absorber (Z) and 

incident photon energy (E) by Z 2 / E. 


http://radiopaedia.org/articles/coherent-scattering


http://radiopaedia.org/articles/coherent-scattering

Photoelectric effect, or photoelectric absorption (PEA) 

is a form of interaction of X-ray or gamma photon with the matter. A low 

energy photon interacts with the electron in the atom and removes it from its 

shell. 

The probability of this effect is maximum when: 

• the energy of the incident photon is equal to or just greater than the 

binding energy of the electron in its shell ('absorption edge') and 

• the electron is tightly bound (as in K shell) 


http://radiopaedia.org/articles/photoelectric-effect

The electron that is removed is then called a photoelectron. The incident 

photon is completely absorbed in the process. Hence it forms one of the 

reason for attenuation of X-ray beam as it passes through the matter. 

PEA is related to the atomic number of the attenuating medium (Z), the 

energy of the incident photon (E) and the physical density of the attenuating 

medium (p) by: Z³ p / E³. 

Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8) 

and if E doubles, PEA will reduce by 8. As small changes in Z can have quite 

profound changes in PEA this has practical implications in the field of 

radiation protection and is why materials with a high Z such as lead (Z = 82) 

are useful shielding materials. 



The incident photon is 

completely absorbed 

in the process. 


Photoelectric effect 

ABSORBED 


Completely Heroes 


Compton effect or Compton scatter is one of three principle 

forms of photon interaction. It is the main cause of scattered radiation in a 

material. It occurs due to the interaction of the X-ray or gamma photon with 

the outermost (and hence loosely bound) valence electron at the atomic level. 

The resultant incident photon gets scattered (changes direction) as well as 

ejects the electron (recoil electron), which further ionizes other atoms. 

Therefore the Compton effect is a partial absorption process and as the 

original photon has lost energy, this is known as Compton shift (the shift 

being a shift of wavelength/frequency). 

Probability of Compton effect: 

• directly proportional to 

• number of outer shell electrons, i.e. the electron density 

• physical density of material 

• inversely proportional to 

• photon energy 

• does not depend on 

• atomic number (unlike photoelectric effect and pair production) 


http://radiopaedia.org/articles/compton-effect

History and etymology 

Named after Professor Arthur Holly Compton (1892- 

1962), US physicist, who was awarded the Nobel Prize 

in Physics in 1927 for his discovery of Compton effect. 



Pair production (PP), like the photoelectric effect, results in the 

complete attenuation of the incident photon. Pair production can only occur if 

the incident photon energy is at least 1.022 MeV. As the photon interacts with 

the strong electric field around the nucleus it undergoes a change of state and 

is transformed into two particles (essentially creating matter from energy): 

■ one electron 

■ one positron (antimatter equivalent of the electron) 

These two particles form the pair referred to in the name of the process. It is 

noteworthy that other 'pairs' of leptons (of which the electron is a type) can be 

created such as muon - antimuon and tau - antitau pairs, however the type of 

lepton pair would dictate the energy of the incident photon necessary to 

create them as both have far higher resting energy masses (1776 MeV for the 

tau and 105 MeV for the muon) than the electron and positron. 


http://radiopaedia.org/articles/pair-production

The reason at least 1.022 MeV of photon energy is necessary is because the 

resting mass (using E=MC² ) of the electron and positron expressed in units 

of energy is 0.511 MeV (or 9.1 x 10 -31 kg) each, therefore unless there is at 

least 0.511 MeV *2 (i.e., 1.022 MeV) it is not possible for the electron-positron 

pair to be created. If the energy of the incident photon is greater than 1.022 

MeV, the excess is shared (although not always equally) between the 

electron and positron as kinetic energy. 

PP is related to the atomic number (Z) of attenuator, incident photon energy 

(E) and physical density (p) by Z E p. 



The electron and positron, once liberated within the medium are dissipated 

through successive interactions within the medium. The electron is quickly 

absorbed, however the fate of the positron is not so straight forward. As it 

comes to a rest, it combines with a neighbouring electron and the two 

particles neutralise each other in a phenomenon known as annihilation 

radiation. Here, the two particles are converted back into two photons of 

electromagnetic radiation, each of 0.511 MeV energy travelling at 180 

degrees to each other (a concept utilised in positron emission tomography - 

PET). These photons are then absorbed or scattered within the medium. 

Pair production in reality does not become the dominant process in water 

below about 30 MeV (due to its dependence on the 'Z' of absorber) and is 

therefore of less importance in the low atomic number soft tissue elements. In 

industrial radiography where high atomic number elements are irradiated, pair 

production can become the major attenuation process assuming the incident 

radiation energy exceeds 1.022 MeV. 





Summary of different mechanisms that cause 

attenuation of an incident x-ray beam 

Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is 

(totally) absorbed, resulting in the ejection of electrons from the outer shell of 

the atom, and hence the ionization of the atom. Subsequently, the ionized 

atom returns to the neutral state with the emission of an x-ray characteristic of 

the atom. This subsequent emission of lower energy photons is generally 

absorbed and does not contribute to (or hinder) the image making process. 

Photoelectron absorption is the dominant process for x-ray absorption up to 

energies of about 500 KeV (

Photoelectric (PE) absorption 

Photoelectric (PE) absorption of x-rays 

occurs when the x-ray photon is (totally) 

absorbed, resulting in the ejection of 

electrons from the outer shell of the 

atom. 


Effect of Photon Energy on Attenuation 

Absorption characteristics will increase or decrease as the energy of the x- 

ray is increased or decreased. Since attenuation characteristics of materials 

are important in the development of contrast in a radiograph, an 

understanding of the relationship between material thickness, absorption 

properties, and photon energy is fundamental to producing a quality 

radiograph. A radiograph with higher contrast will provide greater probability 

of detection of a given discontinuity. An understanding of absorption is also 

necessary when designing x-ray and gamma ray shielding, cabinets, or 

exposure vaults. 

The applet below can be used to investigate the effect that photon energy has 

on the type of interaction that the photon is likely to have with a particle of the 

material (shown in gray). Various materials and material thicknesses may be 

selected and the x-ray energy can be set to produce a range from 1 to 199 

KeV. Notice as various experiments are run with the applets that low energy 

radiation produces predominately photoelectric events and higher energy x- 

rays produce predominately Compton scattering events. Also notice that if the 

energy is too low, none of the radiation penetrates the material. 


This second applet is similar to the one above except that the voltage (KVp) 

for a typical generic x-ray tube source can be selected. The applet displays 

the spectrum of photon energies (without any filtering) that the x-ray source 

produces at the selected voltage. Pressing the "Emit X-ray" button will show 

the interaction that will occur from one photon with an energy within the 

spectrum. Pressing the "Auto" button will show the interactions from a large 

number of photos with energies within the spectrum. 


More Reading on: Photoelectric effect 

The photoelectric effect is the emission of electrons from matter upon the 

absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. 

Upon exposing a metallic surface to electromagnetic radiation that is above 

the threshold frequency or threshold wavelength (absorption edge?) (which is 

specific to the type of surface and material), the photons are absorbed and 

current is produced. 

No electrons are emitted for radiation with a frequency below that of the 

threshold, as the electrons are unable to gain sufficient energy to overcome 

the electrostatic barrier presented by the termination of the crystalline surface. 

By conservation of energy, the energy of the photon is absorbed by the 

electron and, if sufficient, the electron can escape from the material with a 

finite kinetic energy. 

A single photon can only eject a single electron, as the energy of one photon 

may only be absorbed by one electron. 

The electrons that are emitted are often termed photoelectrons. 


https://www.sciencedaily.com/terms/photoelectric_effect.htm

Photoelectric effect, or photoelectric absorption (PEA) 

is a form of interaction of X-ray or gamma photon with the matter. A low 

energy photon interacts with the electron in the atom and removes it from its 

shell. 

The probability of this effect is maximum when: 

• the energy of the incident photon is equal to or just greater than the 

binding energy of the electron in its shell ('absorption edge') and 

• the electron is tightly bound (as in K shell) (?) 



The electron that is removed is then called a photoelectron. The incident 

photon is completely absorbed in the process. Hence it forms one of the 

reason for attenuation of X-ray beam as it passes through the matter. 

PEA is related to the atomic number of the attenuating medium (Z), the 

energy of the incident photon (E) and the physical density of the attenuating 

medium (ρ) by: 

PEA ∝ Z³ ·ρ / E³. 

Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8) 

and if E doubles, PEA will reduce by 8. As small changes in Z can have quite 

profound changes in PEA this has practical implications in the field of 

radiation protection and is why materials with a high Z such as lead (Z = 82) 

are useful shielding materials. 



First principle mechanism of ionization 

The photoelectric effect of ionization involves the complete absorption of the 

photon energy during the process of knocking an electron out of orbit. This 

process primarily occurs with low energy photons ranging in energy between 

10 Kev and less than 500 Kev. (0.01~0.5MeV) 

Notice in the above illustration that an ion pair is created in the interaction 

between the radiation photon and the atom. During this process, when the 

photon liberates the electron, all of the photon s energy is transferred to create 

the ion pair and total absorption has occurred. Remember, there is a binding 

force that the holds the electron in its orbital shell. The amount of energy 

required to create the ion pair must be at least equal to this binding force. 


https://www.nde-ed.org/EducationResources/HighSchool/Radiography/photoelectric_popup.htm

If during the ionization process, only part of the photons energy is needed to 

liberate the electron, the rest of the energy is transferred to the electron in the 

form of speed (velocity). Now that all of the photon's energy is accounted for, 

the photon ceases to exist and total absorption has occurred. Remember that 

a photon is not a particle, but acts like one. When the energy of the photon is 

used, there is nothing left to cause further ionization. 

Keep in mind that electrons orbit in various shells of the atom and not all 

electrons have the same binding energy. This binding energy is dependent 

upon the elements (Z) number and the position of the electron in the atom. 

Those electrons nearer the nucleus possess greater binding energy and will 

require greater photon energy to remove them than will electrons in the outer 

shells. 


https://www.nde-ed.org/EducationResources/HighSchool/Radiography/photoelectric_popup.htm

Photoelectron Spectroscopy 


http://jahschem.wikispaces.com/Photoelectron+Spectroscopy



http://www.met.reading.ac.uk/pplato2/h-flap/phys8_3.html



https://en.wikibooks.org/wiki/Basic_Physics_of_Digital_Radiography/The_Source

Photodisintegration (PD) is the process by which the x-ray photon is 

captured by the nucleus of the atom with the ejection of a particle from the 

nucleus when all the energy of the x-ray is given to the nucleus. Because of 

the enormously high energies involved, this process may be neglected for the 

energies of x-rays used in radiography. (this photodisintegration to be 

distinguishes from photo annihilation of positron/electron pair) 



Thomson scattering (R), also known as Rayleigh, coherent, or classical 

scattering, occurs when the x-ray photon interacts with the whole atom so 

that the photon is scattered with no change in internal energy to the scattering 

atom, nor to the x-ray photon. Thomson scattering is never more than a minor 

contributor to the absorption coefficient. The scattering occurs without the 

loss of energy. Scattering is mainly in the forward direction. 



FIGURE 3. Pulse size as function of voltage in gas ion chamber. 


Ion current chambers have a response magnitude proportional to 

the absorbed energy and are therefore widely used for making dose 

measurements. 

When (1) recombination is negligible, (2) gas amplification does not occur 

and (3) all other charges are efficiently collected, then the steady state current 

produced is an accurate measurement of the rate at which ion pairs are 

formed within the gas. 

Measurement of this ionization current is the principle behind the direct 

current ion chamber. Ion chambers may be constructed of several different 

materials and, because radiation must penetrate the wall of the chamber to 

ionize the gas volume, chambers are chosen for the specific radiation energy 

being evaluated. When considering a particular instrument the energy 

response curve should always be consulted (Fig. 4). Some instruments may 

also have an angular dependence (more sensitivity in some directions), which 

should also be considered when making measurements. Radio frequency 

shielded ionization chambers are available for measurements made near high 

level radio frequency sources. 


FIGURE 3. Pulse size as function of voltage in gas ion chamber. 


FIGURE 4. Energy and directional response of typical ion chamber survey 

meters: (a) example of response curve; (b) comparison of several response 

curves. 


FIGURE 4. Energy and directional response of typical ion chamber survey 

meters: (a) example of response curve; (b) comparison of several response 

curves. 

Legend 






Compton scattering (C) (incoherent scattering) 

occurs when the incident x-ray photon is deflected from its original path by an 

interaction with an electron. The electron gains energy and is ejected from its 

orbital position. The x-ray photon loses energy due to the interaction but 

continues to travel through the material along an altered path. Since the 

scattered x-ray photon has less energy, it, therefore, has a longer wavelength 

than the incident photon. The event is also known as incoherent scattering 

because the photon energy change resulting from an interaction is not always 

orderly and consistent. The energy shift depends on the angle of scattering 

and not on the nature of the scattering medium 



Pair production (PP) can occur when the x-ray photon energy is greater than 

1.02 MeV, but really only becomes significant at energies around 10 MeV. 

Pair production occurs when an electron and positron are created with the 

annihilation of the x-ray photon. Positrons are very short lived and disappear 

(positron annihilation) with the formation of two photons of 0.51 MeV energy. 

Pair production is of particular importance when high-energy photons pass 

through materials of a high atomic number. (single photon for single pair 

production or single high energy photon for multiple pair production?) 



Output Current Measurements 

The ionization current collected in the ion chamber flows through an external 

circuit for measurement. Although in principle an ammeter could be placed in 

the external circuit to read the ion current, in practice the ammeter is not 

placed there, for the current is very small. 

A 440 cm 3 (27 in. 3 ) ion chamber typically produces about 4 × 10 –15 A·μSv –1 

(4 × 10 –14 A·mR –1 ) at standard temperature and pressure. 

A high valued load resistor (on the order of 10 10 Ω) is placed in the circuit and 

the voltage drop across the resistor is measured with a sensitive electrometer. 

A metal oxide silicon field effect transistor (MOSFET) is used in some 

electrometers. The metal oxide silicon field effect transistor produces an input 

impedance on the order of 10 15 Ω to amplify the collected current (Fig. 5). 


FIGURE 5. Operational configuration of current amplifier. 


Vibrating Reed Electrometers 

An alternative approach to ion current measurement is to convert the signal 

from direct current to alternating current at an early stage. This allows a more 

stable amplification of the alternating current signal in subsequent operations. 

The conversion is accomplished in a dynamic capacitor or vibrating reed 

electrometer, by collecting the ion current across a resistive capacitive circuit. 

The capacitance is then changed rapidly, compared to the time constant of 

the circuit. The induced alternating current voltage is proportional to the 

ionization current (Fig. 6). 


FIGURE 6. Principle of vibrating reed electrometer; oscillations of 

capacitance induce alternating current voltage proportional to steady state 

signal current. 


Vibrating Reed Electrometers 

Cary Vibrating Reed Electrometer with Ionization Chamber ( late 1950s) 


https://www.orau.org/PTP/collection/ionchamber/vibratingreedion.htm

Cary Vibrating Reed Electrometer with Ionization Chamber ( late 1950s) 

The spherical ion chamber, 

electrometer head, and amplifier were 

made by Applied Physics Corporation 

of Pasadena, California. The Model 31, 

which replaced the Model 30, was 

introduced in 1957/1958 and seems to 

have been superceded by the Model 32 

in 1959 This system would have been 

used to measure the activity of 

chemically unreactive gases such as 

krypton, xenon, CO2 and HT. Either the 

gas being analyzed would flow through 

the chamber or be held inside the 

chamber for the duration of the 

measurement. 



The weak current generated in the chamber (less than 10 -12 amperes) was 

converted into an alternating current by the vibrating reed in the electrometer 

head. The AC current was then amplified and fed to a strip chart recorder. 

The electrometer readout employed multiple scales and measured up to 30 

volts. 

The spherical ion chamber, shown to the right, is connected directly to the 

electrometer head. Made of stainless steel, it is approximately 3" in diameter 

and has a 250 ml volume. The original version of the chamber was made of 

Pyrex. The reed, a thin metal plate, was vibrated by an electromagnet at 

frequency of 450 cycles per second. The reed also formed part of a capacitor 

onto which the current from the chamber was sent. The cyclical movement of 

the reed resulted in a fluctuating capacitance and the generation of an 

alternating current. The advantage the AC signal had over the original DC 

signal was that the former could be amplified much more reliably. 



An electrometer is an electrical instrument for measuring electric 

charge or electrical potential difference. There are many different types, 

ranging from historical handmade mechanical instruments to high-precision 

electronic devices. Modern electrometers based on vacuum tube or solidstate 

technology can be used to make voltage and charge measurements 

with very low leakage currents, down to 1 femtoampere. A simpler but related 

instrument, the electroscope, works on similar principles but only indicates 

the relative magnitudes of voltages or charges. 


Older electrometers 

Gold-leaf electroscope 

The gold-leaf electroscope was one of the first sensitive instruments used to 

indicate electric charge. It is still used for science demonstrations but has 

been superseded in most applications by electronic measuring instruments. 

The instrument consists of two thin leaves of gold foil suspended from an 

electrode. When the electrode is charged by induction or by contact, the 

leaves acquire similar electric charges and repel each other due to the 

Coulomb force. Their separation is a direct indication of the net charge stored 

on them. On the glass opposite the leaves, pieces of tin foil may be pasted, 

so that when the leaves diverge fully they may discharge into the ground. The 

leaves may be enclosed in a glass envelope to protect them from drafts, and 

the envelope may be evacuated to minimize charge leakage. 

A further cause of charge leakage is ionizing radiation, so to prevent this, the 

electrometer must be surrounded by lead shielding. This principle has been 

used to detect ionizing radiation, as seen in the quartz fibre electrometer and 

Kearny fallout meter. 


This type of electroscope usually acts as an indicator and not a measuring 

device, although it can be calibrated. The Braun[dubious electroscope 

replaced [when?] the gold-leaf electroscope for more accurate measurements. 

The instrument was developed in the 18th century by several researchers, 

among them Abraham Bennet and Alessandro Volta. 



Volta Electrometers

Kolbe electrometer, precision form of goldleaf 

instrument. This has a light pivoted 

aluminum vane hanging next to a vertical 

metal plate. When charged the vane is 

repelled by the plate and hangs at an angle. 


Gold-leaf electroscope 


Modern electrometers 

A modern electrometer is a highly sensitive electronic voltmeter whose input 

impedance is so high that the current flowing into it can be considered, for 

most practical purposes, to be zero. The actual value of input resistance for 

modern electronic electrometers is around 10 14 Ω, compared to around 10 10 Ω 

for nanovoltmeters. Owing to the extremely high input impedance, special 

design considerations must be applied to avoid leakage current such as 

driven shields and special insulation materials. 

Among other applications, electrometers are used in nuclear physics 

experiments as they are able to measure the tiny charges left in matter by the 

passage of ionizing radiation. The most common use for modern 

electrometers is the measurement of radiation with ionization chambers, in 

instruments such as Geiger counters. 


Vibrating reed electrometers 

Vibrating reed electrometers use a variable capacitor formed between a 

moving electrode (in the form of a vibrating reed) and a fixed input electrode. 

As the distance between the two electrodes varies, the capacitance also 

varies and electric charge is forced in and out of the capacitor. The alternating 

current signal produced by the flow of this charge is amplified and used as an 

analogue for the DC voltage applied to the capacitor. The DC input resistance 

of the electrometer is determined solely by the leakage resistance of the 

capacitor, and is typically extremely high, (although its AC input impedance is 

lower). 

For convenience of use, the vibrating reed assembly is often attached by a 

cable to the rest of the electrometer. This allows for a relatively small unit to 

be located near the charge to be measured while the much larger reed-driver 

and amplifier unit can be located wherever it is convenient for the operator. 


Integrating Instruments 

The instruments described above (Fig. 7) are generally rate meters; that is, 

they indicate the radiation at the time of exposure and, depending on its time 

constant, will return to background levels as the source is removed. Some 

instruments may have an integration switch that introduces a capacitor to the 

circuit to accumulate the charge. Leaving such an instrument at an operator’s 

location will indicate the total amount of ionizing radiation that area has 

received, from the time the instrument is engaged. 


FIGURE 7. Examples of ionization chambers located externally on survey 

instruments. Protective caps are removed, showing thin windows for low 

energy X-ray or beta detection. 


Personnel Monitoring Instruments 

Pocket Chambers 

Personnel monitoring instruments, some the size of a ball point pen, are 

usually the integrating type and contain an ionization chamber. One version, 

the pocket chamber, requires the application of an initial charge of 150 to 200 

V by an external instrument. Zero dose is then indicated on a scale contained 

in the charging unit. Exposure of the chamber to ionization decreases the 

initial charge. When the chamber is reconnected to the charging unit the 

reduced charge is translated to the level of exposure (Fig. 8). 


FIGURE 8. Cross section of quartz fiber pocket dosimeter. 

Legend 

1. Low atomic number wall 

2. Graphite coated paper shell 

3. Aluminum terminal head 

4. Aluminum terminal sleeve 

5. Polystyrene support bushing 

6. Central electrode, graphite coated 

7. Polyethylene insulating washer 

8. Polystyrene fixed bushing 

9. Electrode contact 

10. Retaining ring 

11. Aluminum base cap 

12. Polyethylene friction bushing 


Quartz Fiber Pocket Dosimeter. 




https://www.nde-ed.org/EducationResources/CommunityCollege/RadiationSafety/Graphics/DOSE.gif





Pocket Chambers and Pocket Dosimeters 

Paul Frame, Oak Ridge Associated Universities 

Pocket chambers and pocket dosimeters are small ionization chambers that, 

as the name implies, are usually worn in the pocket. While they were 

designed to measure x-rays and gamma ray exposures, they would also 

respond to betas above 1 MeV. Neutron-sensitive versions were also 

available. The terms pocket chamber and pocket dosimeter are often used 

interchangeably. The original distinction between the two terms, used here, is 

rarely made anymore. In part, this is due to the fact that the devices that I call 

pocket chambers are rarely used any more. 

Note: 

pocket dosimeter is an ion chamber type. 


http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm




1. Pocket Chambers 

Pocket chambers go by a variety of names: indirect-reading dosimeters, nonself-reading 

dosimeters and condenser-type pocket dosimeters. Prior to WW 

II, they were only used to a limited extent, primarily in medical facilities and 

around accelerators. The Manhattan Project however created a huge demand 

and they were worn by almost everyone who might be exposed to radiation. 

A pocket chamber acts as an air-filled condenser (capacitor) much like the 

thimble chambers used in radiology. Prior to being worn, it is given a charge 

with a charger-reader, e.g., the Victoreen Minometer. Any subsequent 

exposure to radiation ionizes the air inside the chamber and this reduces the 

stored charge. In order to quantify the exposure, the charge is measured and 

the decrease is related to the exposure. 



Manhattan Project 














Manhattan Project Director 

J Robert Oppenheimer 


WWII 


Ion Chamber Charger 

& Reader 

PP-630(A)/PD Dosimeter Charger 

(ca. 1954-1961) 

The PP-630 (A)/PD Dosimeter 

Charger is the military equivalent of 

the Keleket Model 430A chargerreader 

which was advertised as 

early as 1954. Since this PP-630(A) 

has "August 29 1961" stamped on it, 

I assume that it probably dates from 

1954 to 1961. Unfortunately I have 

not been able to locate any specific 

references to the PP-630A/PD. 


http://www.orau.org/ptp/collection/radiac/PP630.htm

Ion Chamber Charger & Reader 

Radiac Computer-Indicator CP-95A/PD is designed as a portable radiac computer-indicator, for computing & indicating the total amount of X and gamma radiation to which Radiac Detector 

DT-60()/PD has been exposed (and thus revealing the X and gamma radiation to which the wearer of the DT-60()/PD had been exposed). The CP-95A/PD operates in conjunction with 

Radiac Detector DT-60()/PD (not supplied) which contains a specially compounded silver-actuated phosphor glass. When the total radiation dosage of a DT-60()/PD is to be measured, the 

DT-60()/D is placed in Radiac Computer-Indicator C-95A/PD and exposed to a source of ultraviolet light. The ultraviolet light causes the silver-activated glass to emit an orange 

luminescence, the intensity of which is proportional to the total amount of radiation the glass has received. The intensity of the orange luminescence is measured by a photomultiplier tube 

which is fitted with a filter to prohibit the passage of blue and green light. The photomultiplier tube employs the principle of secondary emission to amplify the initial electron emission caused 

by the filtered orange illumination of the light-sensitive cathode. The output of the photomultiplier tube is applied to an indicating circuit to indicate the total amount of radiation to which the 

wearer of the DT60()/PD has been exposed. 


http://www.orau.org/ptp/collection/radiac/CP95APD.htm

Pocket chambers were approximately 4 - 5" long and 0.5" in diameter. An 

aluminum rod (ca. 0.0625” in diameter) running along the chamber axis 

served as one electrode, while the outer wall of the chamber served as the 

other electrode. The central electrode was suspended at each end with a 

polystyrene insulator and at one end it penetrated the insulator to serve as 

the charging contact. One problem with the early models involved the 

threaded caps that were used to protect the charging contact - they would 

wear and the metal fragments would get on the insulator. The graphite 

coating on the inside of the chamber wall caused a similar type of problem 

with some of the early models because it would sometimes flake off and short 

out the chamber. The early models were also susceptible to discharge as a 

result of mechanical shock because the central electrode would flex and 

contact the chamber wall. To solve this problem, later versions used a thicker 

central electrode and/or positioned a small insulating disk in the center of the 

electrode. Because of these problems, it was usual for a worker to wear two 

dosimeters and the lower of the two readings was considered the most 

accurate. 






2. Pocket Dosimeters (quartz fiber electroscopes) 

Like pocket chambers, pocket dosimeters are known by a number of other 

names, e.g., direct-reading dosimeters, self-reading pocket dosimeters and 

pocket electroscopes. They are actually quartz fiber electroscopes the 

sensing element of which is a movable bow-shaped quartz fiber that is 

attached at each end to a fixed post. The latter is also shaped like a bow (or 

horseshoe). The dose is determined by looking through the eyepiece on one 

end of the dosimeter, pointing the other end towards a light source, and 

noting the position of the fiber on a scale. Until 1950 or so, the vast majority of 

pocket dosimeters had a range up to 200 mR, although a few high range 

versions were available for emergency situations. Higher range versions 

became more readily available in the 1950s for military and civil defense 

purposes. 



Pocket dosimeters tended to be slightly larger than pocket chambers. Their 

walls might be made of aluminum, bakelite, or some other type of plastic. If 

the material was not conductive, the inner surface of the chamber was coated 

with Aquadag (graphite). The central electrode was usually a phosphor 

bronze rod. This made pocket dosimeters more energy dependent than 

pocket chambers whose central electrodes were usually aluminum. Some 

dosimeters (e.g., Keleket Model K-145) employed boron-lined chambers 

which made them sensitive to thermal neutrons. 

Pocket dosimeters must be charged (ca. 150 ~ 200 volts) with some sort of 

charger, but they do not require another device to read them. This allows the 

worker to determine his or her exposure at any time, an important advantage 

when working in high radiation fields. 

The first direct reading pocket dosimeters were built by Charlie Lauritsen at 

the California Institute of Technology. 



3. Pocket Chambers (indirect-reading) vs Pocket Dosimeters (direct 

reading) 

1. Pocket chambers were far less expensive (ca. $5 vs $40 in 1950) 

2. Pocket chambers, despite their problems, were more reliable. 

3. Pocket chambers did not permit the wearer to know their exposure, for 

military purposes, this was sometimes desirable. 

4. Pocket dosimeters allowed the worker to check their exposure during a 

particular task and to take corrective actions when appropriate. 

5. Pocket dosimeters did not have to be recharged every time they were read. 

6. Pocket dosimeters could use very small chargers, small enough to easily 

fit into a pocket. 




CDV-750 Dosimeter Charger and THREE (3) CDV-742 Dosimeters 

$260.00 The direct-reading pocket dosimeter is a portable instrument designed to measure the total dose of moderate and 

high levels of gamma radiation. The instruments make use of a small quartz fiber electroscope as an exposure detector and 

indicator. An image of the fiber is projected onto a film scale and viewed through the eyepiece lens. The scale is calibrated in 

roentgens (R) and may be read by looking through the eyepiece toward a lamp or other source of light. A CDV-750 dosimeter 

charger must be used in conjunction with the dosimeter to set the instrument to zero. The charger may also be used to read the 

scale or you can hold the dosimeter up to any light source and look through it. 

NOTE: the CDV-750 uses 1 D cell Battery (not included). The CDV-742 Dosimeter is an 'electroscope' it is electro statically 

charged by the CDV-750 charger. The Dosimeter does NOT use a battery. No battery to replace, no battery to go bad, EVER! 


http://josephdanielassociates.us/index.php?main_page=product_info&products_id=356













WWII Heroes 


Direct Reading DosimetersThe direct reading dosimeter operates on the 

principle of the gold leaf electroscope (Fig. 9). A quartz fiber is displaced 

electrostatically by charging it to a potential of about 200 V. An image of the 

fiber is focused on a scale and viewedthrough a lens at one end of the 

instrument. Radiationexposure of the dosimeter discharges the fiber, 

allowing itto return to its original position.Personnel dosimeters may have a 

full scale reading of 1 to 50 mSv (100 mR to 5 R) (0.1R ~ 5R) and may have 

other scales according to applicable regulations. Chambers are available with 

thin walls for sensitivity to beta radiation over 1 MeV and may be coated on 

the inside with boron for neutron sensitivity. Figure 10 demonstrates the 

energy response of selfreading pocket dosimeters. Table 2 lists 

performance specifications of dosimeters in general. 

Keypoints: 

■ Chambers are available with thin walls for sensitivity to beta radiation 

1 MeV 

■ may be coated on the inside with boron for neutron sensitivity. 


FIGURE 8. Cross section of quartz fiber pocket dosimeter. 


FIGURE 9. Cross section of pocket (direct reading) ionization chamber. 


FIGURE 10. Energy dependence of response of different commercial selfreading 

dosimeters. 


TABLE 2. General performance specifications for dosimeters. 


Proportional Counters 

If the electric field in an ion chamber is raised above the ionization potential 

but below saturation potential, enough energy is imparted to the ions for 

production of secondary electrons by collision and gas amplification. 

Operation at this electric potential overcomes the difficulty of the small 

currents in the ionization region yet takes advantage of pulse size 

dependence for separating various ionizing energies. 

When an ionization chamber is operated in this region it is called a 

proportional counter. 

The size of the output pulse is determined by, and proportional to, the number 

of electrons collected at the anode (n) and the voltage applied (V) at the 

detector (Output∝n·V) . By careful selection of gases and voltages, a 

properly designed proportional counter can detect alphas in the presence of 

betas, or higher energy beta and gamma radiation in the presence of lower 

energies. Proportional counters are often used in X-ray diffraction 

applications. 


REM (Roentgen equivalent man) – Dose Equivalent 

One of the two standard units used to measure the dose equivalent (or 

effective dose), which combines the amount of energy (from any type of 

ionizing radiation that is deposited in human tissue), along with the medical 

effects of the given type of radiation. For beta and gamma radiation, the dose 

equivalent is the same as the absorbed dose. By contrast, the dose 

equivalent is larger than the absorbed dose for alpha and neutron radiation, 

because these types of radiation are more damaging to the human body. 

Thus, the dose equivalent (in rems) is equal to the absorbed dose (in rads) 

multiplied by the quality factor of the type of radiation [see Title 10, Section 

20.1004, of the Code of Federal Regulations (10 CFR 20.1004), "Units of 

Radiation Dose"]. The related international system unit is the sievert (Sv), 

where 100 rem is equivalent to 1 Sv. For additional information, see Doses in 

Our Daily Lives and Measuring Radiation. 

先有 rad 才有 rem / 先有 Gray 才有 Sievert 


http://www.nrc.gov/reading-rm/basic-ref/glossary/rem-roentgen-equivalent-man.html

Townsend Discharges (Avalanches) 

The Townsend discharge or Townsend avalanche is a gas ionisation process 

where free electrons are accelerated by an electric field, collide with gas 

molecules, and consequently free additional electrons. Those electrons are in 

turn accelerated and free additional electrons. The result is an avalanche 

multiplication that permits electrical conduction through the gas. The 

discharge requires a source of free electrons and a significant electric field; 

without both, the phenomenon does not occur. 

The Townsend discharge is named after John Sealy Townsend, who 

discovered the fundamental ionisation mechanism by his work between 1897 

and 1901. 


https://en.wikipedia.org/wiki/Townsend_avalanche

Sir John Sealy Townsend 



General description of the phenomenon 

The avalanche occurs in a gaseous medium that can be ionised (such as air). 

The electric field and the mean free path of the electron must allow free 

electrons to acquire an energy level (velocity) that can cause impact 

ionisation. If the electric field is too small, then the electrons do not acquire 

enough energy. If the mean free path is too short, the electron gives up its 

acquired energy in a series of non-ionising collisions. If the mean free path is 

too long, then the electron reaches the anode before colliding with another 

molecule. 

A positive charge q is placed in a uniform 

electric field E set up between two charged 

parallel plates. If the particle is at a 

distance s from the negative plate, its 

electrical potential energy is qEs joules 

(C·N·C-1·m = N.m) 

http://www.physchem.co.za/OB11-ele/charge3.htm 



Note: 

• If the electric field is too small, then the electrons do not acquire enough 

energy. 

• If the mean free path is too small, then the electrons do not acquire 

enough kinetic energy. 

• If the mean free path is too short, the electron collides too early with low 

energy, gives up its acquired energy in a series of non-ionising collisions. 

• If the mean free path is too long, then the electron reaches the anode 

before colliding with another molecule. 



The avalanche mechanism is shown in the accompanying diagram. The 

electric field is applied across a gaseous medium; initial ions are created with 

ionising radiation (for example, cosmic rays, X radiation and gamma ray). An 

original ionisation event produces an ion pair; the positive ion accelerates 

towards the cathode while the free electron accelerates towards the anode. If 

the electric field is strong enough, the free electron can gain sufficient velocity 

(energy) (qEs) to liberate another electron when it next collides with a 

molecule. 



The two free electrons then travel towards the anode and gain sufficient 

energy from the electric field to cause further impact ionisations, and so on. 

This process is effectively a chain reaction that generates free electrons. 

The total number of electrons reaching the anode is equal to the number of 

collisions, plus the single initiating free electron. (n+1) Initially, the number of 

collisions grows exponentially. (?) The limit to the multiplication in an electron 

avalanche is known as the Raether limit. 

The Townsend avalanche can have a large range of current densities. In 

common gas-filled tubes, such as those used as gaseous ionisation detectors, 

magnitudes of currents flowing during this process can range from about 

10 −18 amperes to about 10 −5 amperes. 



Visualization of Proportional Counter Gas Magnification Event 

Single gas 

avalanche near 

the anode? 



Visualization of Proportional Counter Gas Magnification Event 

Single gas 


the anode? 



Multiple Gas Amplification due to multiple ionization 

Single gas 


the anode 


dose equivalent (in rems) is equal to the absorbed dose (in rads) 

multiplied by the quality factor of the type of radiation 

rems 

rads 


http://www.nrc.gov/reading-rm/basic-ref/glossary/rem-roentgen-equivalent-man.html

PART 3. Geiger-Müller Counters 

Operating Voltage Level 

Increasing voltage beyond the proportional region (Fig. 3) will eventually 

cause the gas avalanche to extend along the entire length of the anode wire. 

When this happens, the end of the proportionalregion is reached and the 

geiger-müller region begins. An instrument operating in this voltage range, 

using a sealed gas filled detector, is referred to as a geiger-müller counter, a 

GM counter or simply a geiger tube. This instrument was introduced in 1928 

and its simplicity and low cost have made it the most popular radiation 

detector since then. Geiger-müller counters complement the ion chamber and 

proportional counter and comprise the third category of gas filled 

detectors based on ionization. 


Operating Voltage Level 


Visualization of Geiger Muller Gas Magnification Event- Townsend 

Avalanches 

Multiple gas 

avalanche 


Visualization of Geiger Muller Gas Magnification Event- Townsend 

Avalanches 

Visualization of the spread of 

Townsend avalanches by means 

of UV photons. This mechanism 

allows a single ionising event to 

ionise all the gas surrounding the 

anode by triggering multiple 

avalanches. 


Properties 

Extension of the gas avalanche increases the gas amplification factors so that 

10 9 to 10 10 ion pairs are formed in the discharge. This results in an output 

pulse large enough (0.25 to 10 V) to require no sophisticated electronic 

amplification circuitry for readout. At this voltage, the size of all pulses, 

regardless of the nature of the ionization, is the same. 

When operated in the geiger-müller region, a counter cannot distinguish 

among the several types of radiation and therefore is not useful for 

spectroscopy or for the detection of one energy event in the presence of 

another. An external shield is often used to filter out alpha and beta particles 

in the presence of gamma energies. 

Note: 

The amplification factor: 10 9 to 10 10 ion pairs are formed in the discharge for 

single ionization of multiple events avalanches 


Resolving Time 

As an ionizing event occurs in the counter, the avalanche of ions paralyzes 

the counter. The counter is then incapable of responding to another event 

until the discharge dissipates and proper potential is established. 

The time it takes to reestablish the electric field intensity is referred to as the 

resolving time. 

Average resolving time for a geiger-müller counter is about 100 ms, which 

must be corrected at high level readings. Resolving time τ of a counter may 

be determined by counting two sources independently (R 1 and R 2 ), then 

together (R 1 , R 2 ). The background count is R b . 


Correct counting rate R can be calculated from observed counting rate R o 

andresolving time τ in the following equation for nonparalyzable systems: 


Dead Time 

The relationship of resolving time to dead time and recovery is illustrated in 

Fig. 11.Resolving time may be a function of the detector alone or of the 

detector and its signal processing electronics. Its effect on the real counting 

rate depends on whether the system design is paralyzableor nonparalyzable. 


FIGURE 11. Resolving time, dead time and recovery time for geiger-müller 

system. 


Nonparalyzable Systems 

In Fig. 12, a time scale is shown indicating six randomly spaced events in the 

detector. At the bottom of the illustration is the corresponding dead time 

behavior of a detector assumed to be nonparalyzable. A fixed time τ follows 

each event that occurs during the live period of the detector. Events occurring 

during the dead time have no effect on the detector, which would record four 

counts from the six interactions. 


FIGURE 12. Processing of detector interactions in paralyzable 

and nonparalyzable systems. 


Paralyzable Systems The top line of Fig. 12 illustrates a paralyzable system. 

Resolving time τ follows each interaction, whether it is recorded or not. 

Events that occur during resolving time τ are not recorded and further extend 

the dead time by another period τ. The chart indicates only three recorded 

events from the six interactions. In this case, τ increases with increased 

number of interactions. It can be demonstrated that with a paralyzable system 

(at increasingly higher interaction rates), the observed counting rates can 

actually decrease with an increased number of events. When using a 

counting system that may be paralyzable, extreme caution must be taken to 

ensure that low observed counting rates correspond to low interaction rates, 

rather than very high interaction rates with accompanying, long dead time. It 

is possible for a paralyzable system to record the first interaction and then be 

paralyzed, recording zero counts in high radiation fields. 


Quenching 

As positive ions are collected after a pulse, they give up their kinetic energy 

by striking the wall of the tube; Ultraviolet photons and/or electrons are 

liberated, producing spurious counts. Prevention of such counts is called 

quenching. Quenching may be accomplished electronically (by lowering the 

anode voltage after a pulse) or chemically (by using a self-quenching gas). 

■ Electronic Quenching (by lowering the anode voltage after a pulse) 

Electronic quenching is accomplished by introducing a high value of 

resistance into the voltage circuit. This will drop the anode potential until all 

the positive ions have been collected. 


■ Self-Quenching Gas (absorb ultraviolet (UV) photons) 

A self-quenching gas is one that can absorb ultraviolet (UV) photons without 

becoming ionized. One way to use this characteristic is to introduce a small 

amount of organic vapor, such as alcohol or ether, into the tube. The energy 

from the ultraviolet photons is then dissipated by dissociating the gas 

molecule. Such a tube is useful only as long as it has a sufficient number of 

organic molecules to dissociate, generally about 10 8 counts. 

To avoid the problem of limited lifetime, some tubes use halogens (chlorine or 

bromine) as the quench gas. The halogen molecules also dissociate in the 

quenching process but they are replenished by spontaneous recombination at 

a later time. Halogen quench tubes have an infinite lifetime and are preferred 

for extended applications. 

Reaction products of the discharge often produce contamination of the gas or 

deposition on the anode surface and generally limit the lifetimes of geigermüller 

tubes. 


FIGURE 13. Assortment of geiger-müller counters demonstrating availability 

of sizes and shapes. Smallest counter shown isabout 30 mm (1 in.) long. 


Geiger Muller 


http://solderpad.com/solderpad/mightyohm-geiger-counter/board/embed

Geiger Muller 


http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html

Geiger Muller 



Geiger Muller 



Geiger Muller 



Geiger Muller 



Geiger Muller 



Geiger Muller 



Smart Geiger Counter 


http://akihabaranews.com/2015/10/05/article-en/app-enabled-geiger-counter-smartphones-113561601

Design Variations 

Geiger-müller counters (Fig. 13) are available in various shapes and sizes. 

The most common form is that of a cylinder with a central anode wire. If low 

energy beta or alpha particles are to be counted, a unit with a thin entrance 

window (1 to 4 mg·cm –2 ) should be selected. For surveying large surfaces, 

pancake or large window counters are available. 

High count rate instruments, greater than 0.14 mSv·s –1 (50 mR·h –1 ), generally 

contain a small tube to minimize resolving time of the system; large volume 

detectors may require significant correction. 

Note: small volume minimize resolving time 

A geiger-müller counter response to gamma rays occurs by way of gamma 

ray interaction with the solid wall of the tube. The incident gamma ray 

interacts with the wall and produces a secondary electron that subsequently 

reaches the gas. The probability of gamma ray interaction generally increases 

with higher density wall material. 


A geiger-müller counter response to gamma rays occurs by way of gamma 

ray interaction with the solid wall of the tube. The incident gamma ray 

interacts with the wall and produces a secondary electron that subsequently 

reaches the gas. The probability of gamma ray interaction generally increases 

with higher density wall material. 

Note: 

■ Smaller chamber volume, minimize resolving time 

■ Gamma ray interact with the chamber wall 

■ Higher the density higher the probability of interaction 

■ Higher the atomic number higher the interaction 


FIGURE 14. Dose rate ratio versus effective energy for 

personnel radiation monitor. 


FIGURE 14. Dose rate ratio versus effective energy for 

personnel radiation monitor. 

Ir-192 


TABLE 1. Characteristics of three isotope sources commonly used for 

radiography. 


Alarming Rate Meters (Personnel Monitors) 

Small geiger-müller tubes are used in pocket-sized units for personnel 

monitoring. They generally emit a high frequency chirp at a rate proportional 

to the subjected dose rate. United States regulations specify an alarm 

threshold of 500 mSv·h –1 (500 mR·h –1 ) (500 μR·h–1) for field gamma 

radiography. The energy dependence curve for one such instrument is shown 

in Fig. 14. 


Mini Geiger Muller Alarm Meter 






Smart Geiger Counter 


http://xronosclock.com/home/?p=4238




Applications 

Geiger-müller counters are the most widely used, general purpose radiation 

survey instruments. It must be remembered that geiger-müller counters, 

unlike current ionization chambers, read pulses (regardless of their energy or 

ionizing potential) and register in counts per minute. Some instruments have 

a scale calibrated in milliroentgens per hour (mR·h –1 ); however, this is an 

arbitrary scale calibrated on the radiation from radium-226, cesium-137 or 

some other energy (Fig. 15). Another scale is microsieverts per second 

(μSv·s –1 ). A sensitivity versus energy table should always be consulted 

before making measurements with a geiger-müller instrument. 


FIGURE 15. Typical energy response curves for geiger-müller counters 

(a) shielded versus unshielded; 

(b) radiation incident on side versus front; 

(c) exposure ratio close to ideal with radiation incident normal to long axis of 

probe; 

(d) radiation incident normal to long axis of probe. 


(a) shielded versus unshielded; 


(b) radiation incident on side versus front; 


(c) exposure ratio close to ideal with radiation incident normal to long axis of 

probe; 


(d) radiation incident normal to long axis of probe. 


PART 4. Scintillation Detectors 

Soon after the discovery of X-rays and radioactivity, it was observed that 

certain materials emit visible light photons after interacting with ionizing 

radiation. These light photons appear to flash or sparkle and the materials are 

said to scintillate. Scintillators commonly used with radiation survey 

instruments are solid materials. Being denser than gases, these scintillators 

have greater detection efficiencies and are useful for low level measurements. 

For gamma photons, scintillators have detection efficiencies 10 6 times greater 

than typical gas ionization chambers. Detection of alpha and beta particles, 

neutrons and gamma photons is possible with various scintillator systems 

(Table 3). 


TABLE 3. Common scintillators. 


Scintillation Process 

Radiation interactions with matter produce excitation as well as irrigation. 

Ionization refers to the removal of an electron from an atom and excitation 

refers to the elevation of an electron’s energy state. The return of excited 

electrons to their normal, lower energy state is called deexcitation. 

Scintillators excited by ionizing radiation return to lower energy states quickly 

and emit visible light during the deexcitation process. Radiation detection is 

possible by measuring the scintillator’s light output (Fig. 16). 


FIGURE 16. Energy diagram of scintillation process. 


Materials and Characteristics 

Scintillation materials come in gaseous, liquid and solid forms. Organic liquids 

and solids, as well as inorganic gases and solids, are common scintillators. 

Organic, solid scintillators are available as crystals, plastics and gels. 

Inorganic solid scintillators are usually alkali halide crystals. 

The scintillation process in inorganic materials requires the presence of small 

amounts of an impurity, or activator. Inorganic solid scintillators are commonly 

used with radiation survey instruments and are listed in Table 3. 


Desirable Scintillator Characteristics 

A useful and practical scintillator needs to have most of the characteristics 

listed below. Not all of these characteristics are ideally satisfied by each 

scintillator and often a compromise is acceptable. 

1. The scintillator should be of high density and large enough to ensure 

adequate interaction with the ionizing radiation. 

2. Efficient conversion of the electron’s kinetic energy into visible light is 

required and the light yield should be linearly related to the deposited 

electron kinetic energy. 

3. The scintillator should be of good optical quality, transparent to its emitted 

light and free of hydroscopic effects, and should have an index of 

refraction close to that of glass. 

4. The wavelength of the emitted light should be appropriate for matching to 

a photomultiplier tube. 


Photomultiplier Tubes 

Before the advent of photomultiplier tubes (PMTs), scintillation light photons 

had to be visually counted. This limited the use and development of 

scintillators. In the 1940s, the photomultiplier tube was developed and 

dramatically increased the use of scintillators, to the point where scintillators 

are preferred over other radiation detectors for many survey applications. The 

photomultiplier tube’s function is to convert the scintillator’s light output into a 

electrical pulse. The photomultiplier tube is composed of a photosensitive 

layer, called the photocathode, and a number of electron multiplication 

structures called dynodes. Conversion of the scintillation light into 

photoelectrons is accomplished by the photocathodes through the 

photoelectric effect. To maximize the information contained in the scintillation 

light, the photomultiplier tube photocathode should be matched to the 

scintillator; the scintillator and photomultiplier tube should be optically coupled 

to minimize light losses. 


Electron multiplication, or gain, is accomplished by positively charging the 

dynodes in successive stages, so that the total voltage applied to the 

photomultiplier tube is around 1000 V. Electrons emitted by the photocathode 

are focused toward the first dynode; more electrons are emitted than were 

initially incident on the dynode. This is repeated at each dynode stage. The 

photocathode and dynodes are positioned in a glass enclosed vacuum so 

that air molecules will not interfere with the collection of electrons. The net 

result of the photomultiplier tube may be an electron gain up to 10 10 per 

emitted photoelectron. Figure 17 illustrates the structure of a photomultiplier 

tube. 


FIGURE 17. Cutaway drawing of photomultiplier tube, showing crystal, 

photocathode, collecting dynodes and voltage divider network. 


PMT 


PMT 


PMT 


PMT - Photomultiplier tubes lining the walls of the Daya Bay neutrino detector. 

The tubes are designed to amplify and record the faint flashes of light that 

signify an antineutrino interaction. This experiment aims to measure the final 

unknown mixing angle that describes how neutrinos oscillate — another 

chapter in Brookhaven National Laboratory's long history of neutrino research 

over the last several decades. 


https://www.bnl.gov/newsroom/news.php?a=24055

System Electronics 

Once the output pulse from a photomultiplier tube is generated, it is amplified 

and analyzed. The pulse height, or amplitude, is proportional to the amount of 

energy deposited within the scintillator and can be correlated to a count rate 

or scale of microsievert per second (μSv·s –1 ) or milliroentgen per hour 

(mR·h –1 ) when calibrated against a known energy source. (See Fig. 18.) 


FIGURE 18. Comparison of sodium iodide (thallium activated) and 

germanium detectors for gamma spectroscopy. 


Energy (MeV)

PART 5. Luminescent Dosimetry 

Thermoluminescent Dosimetry 

Thermoluminescence is the emission of light from previously irradiated 

materials after gentle heating. The radiation effect in thermoluminescent (TL) 

materials is similar to that observed in scintillators, except that light photon 

emission does not occur in thermoluminescent materials until some heat 

energy is supplied (Fig. 19). Measurement of the light photons emitted after 

heating permits correlation to the amount of ionizing radiation energy that was 

absorbed in the thermoluminescent material. 

Thermoluminescent dosimetry (TLD) is possible for beta, gamma and neutron 

(alpha?) radiations, if the appropriate thermoluminescent material is used. 


Thermoluminescence 


Thermoluminescence 


Lithium Fluoride Properties 

The most common thermoluminescent phosphor used in gamma and neutron 

personnel dosimetry is lithium fluoride ( 6 3LiF for neutron detection)) . 

Other thermoluminescent phosphors are available for personnel dosimetry 

but, for various reasons, are not as well suited as lithium fluoride. The 

advantages of lithium fluoride include its: 

1. usefulness over a wide dose range, 

2. linear dose response, 

3. near dose rate independence, 

4. reusability, 

5. stability, 

6. short readout time and 

7. near tissue equivalence. 


Disadvantages include the loss of information after readout and lack of 

information about the incident radiation energy. 

Both gamma photons and neutrons produce ionization indirectly. Gamma 

photons interact with matter, releasing electrons that in turn cause ionization. 

Lithium fluoride undergoes interactions with gamma photons and is therefore 

used in gamma dosimetry. 

Slow neutrons require the presence of the lithium fluoride enriched with 

lithium-6 for detection of the (n, α) nuclear reaction. 

6 

3 Li + n → 4 2 He + 2 1 D + γ 


Fast neutron detection with lithium fluoride would only be possible if the fast 

neutrons were slowed down to thermal energies before reaching the lithium 

fluoride thermoluminescent dosimeter. Nearly complete elimination of neutron 

response in lithium fluoride is possible with lithium fluoride enriched with 

lithium-7. In a mixed gamma and slow neutron field, distinction of gamma and 

neutron doses is possible by comparing the readings of two lithium fluoride 

thermoluminescent dosimeters with different lithium-6 contents. 

Keywords: 

• Nearly complete elimination of neutron response in lithium fluoride is 

possible with lithium fluoride enriched with lithium-7. 

• In a mixed gamma and slow neutron field, distinction of gamma and 

neutron doses is possible by comparing the readings of two lithium 

fluoride thermoluminescent dosimeters with different lithium-6 contents. 


Thermoluminescent Dosimetric Readout Systems 

Thermoluminescent dosimetric readout systems are commonly made up of a 

sample holder, heating system, photomultiplier tube (light detector), high 

voltage supply, signal amplifier and a recording instrument. The 

thermoluminescent dosimetric sample is heated indirectly, using electrical 

resistance heat applied to a pan or planchette. The photomultiplier tube 

converts the light output into an electronic pulse that is then amplified before 

recording. The recording instrument may be a plotter or any other instrument 

that can measure the amplified photomultiplier tube output signal. A plot of 

the output signal versus time is equivalent to emitted light intensity versus 

heat and results in a glow curve. 

The area under the glow curve is proportional to the absorbed dose (Fig. 20). 

Uses of thermoluminescent measurement of radiation include personnel 

dosimetry, medical dosimetry, environmental monitoring and archeological 

and geological dating. 


FIGURE 19. Thermoluminescence process.5 


FIGURE 20. Typical glow curve. Integrated area under curve is measure of 

radiation exposure. 


Optically Stimulated Luminescence Dosimetry 

Optically stimulated luminescence dosimeters typically have aluminum oxide 

detectors and are available in plastic holders, or body badges, that are worn 

at collar level to measure full body dose. They can measure gamma ray and 

X-ray doses from 10μSv to 10Sv (1 mrem to 1 krem). 


TDL 


http://www.rpe.org.in/article.asp?issn=0972-0464;year=2011;volume=34;issue=1;spage=6;epage=16;aulast=Bhatt;type=3

TDL 



TDL 



TDL 



PART 6. Neutron Detection 

Characteristics 

The neutron is a part of the nucleus, has no charge and is somewhat larger in 

mass than the proton. It is similar to the photon in that it has no charge and 

produces ionization indirectly; it is different from the photon because it is a 

nuclear particle and not a unit of electromagnetic energy. Because the 

neutron is an uncharged particle, its interactions with matter are different from 

those of charged particles or photons. Ionization by neutrons is indirect: as a 

result of neutron interactions with matter, recoil nuclei, photons or charged 

particles are produced and then interact with matter by various mechanisms 

that cause ionization. 


Neutron Sources 

Neutrons are classified according to their energies as shown in Table 4. 

Some radionuclides (such as californium-252) may decay by 

spontaneousfission and emit neutrons with fission fragments, photons and 

electrons. 

Induced fission reactions, such as those occurring in a nuclear reactor with 

uranium, emit about 2.5 neutrons per fission. Fission neutrons range in 

energy from 0.025 eV to about 16 MeV. 

Other neutron sources are the result of various nuclear reactions and produce 

either a spectrum of neutron energies or monoenergetic neutrons. Common 

neutron producing nuclear reactions are the (γ, n), (α, n), (p, n), (d, n) and (α, 

2n) reactions and may use radionuclide emissions or accelerated particles to 

initiate the reaction. Neutron radiography usually uses radionuclides that emit 

alpha or gamma photons and produce neutrons by (α, n) and (γ, n) reactions 

with various target materials. 


TABLE 4. Neutron classification. 

Class 

Thermal 

Epithermal 

Slow 

Intermediate 

Fast 

Relativistic greater than 

Energy 

< 0.3 meV 

>1 eV 

0 meV to 100 eV 

100 eV to 10 keV 

10 keV to 10 MeV 

10 MeV 


Neutron Detectors 

There are several mechanisms and devices used to detect neutrons of 

various energies. Ionization chambers, proportional counters, scintillators, 

activation foils, track etch detectors, film emulsions, nuclear emulsions and 

thermoluminescent phosphors are some of the many devices used to detect 

neutrons. The main mechanisms used to detect neutrons in these devices are 

the (n, α), (n, p), (n, d), (n, f) and (n, γ) nuclear reactions. 

(n, f ) f= fragments 

Proportional Neutron Detectors Many fast and slow neutron counters use 

proportional counting chambers filled with boron trifluoride (BF 3 ) gas, often 

enriched in boron-10. The interaction of thermal (slow) neutrons with boron 

gas releases an alpha particle of several mega electron volts that is easily 

detected in the proportional mode. Fast neutrons are detected by a similar 

counter, in which thermal neutrons are absorbed in an external cadmium 

shield; the fast neutrons that pass through the shield are thermalized in 

hydrogen rich material and counted in the proportional chambers. 

10 

5 B + n → 4 2 He2+ + 7 3 Li + γ 


Scintillation 

Scintillators containing lithium-6, boron-10 and hydrogenous plastics have 

been used as neutron detectors. Lithium-6 is used as lithium iodide (europium 

activated) and in lithium glasses to detect slow and fast neutrons. Scintillators 

loaded with boron-10 are used for slow neutron detection. Plastic scintillators 

with high hydrogen content are used in fast neutron detection and 

spectroscopy by measuring the energy deposited by recoil protons. 

Activation Foils Introducing certain materials to an incident neutron flux will 

result in these materials becoming radioactive. The process is called 

activation and gaining information about the incident neutron flux and energy 

is possible by analyzing the radiations emitted from the activated foil. 

Activation foils rely on (n, γ), (n, p), (n, α), (n, f ) and other nuclear reactions 

to cause the activation. Selection of the proper activation foil can give a rough 

estimate of the neutron energy spectrum. In high neutron flux fields, where 

instruments would fail, activation foils are used as integrating detectors. 


Miscellaneous Neutron Detectors Track etch detectors, nuclear 

emulsions and film have all been used to detect neutrons. Various neutron 

interactions with the detector material or foils in intimate contact with the 

detectors allow these systems to operate as integrating dosimeters. 


TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials 

Material 

Useful Reactions 

Cross Section for 

Life 

Thermal Neutrons (barns) 

Lithium 

6 Li(n,α) 3 H 

910 

prompt 

Boron 

10 B(n,α) 7 Li 

3,830 

prompt 

Rhodium 

103 Rh(n) 104m Rh 

11 

45 min 

103 

Rh(n) 104 Rh 

139 

42 s 

Silver 

107 Ag(n) 108 Ag 

35 

2.3 min 

109 Ag (n) 110 Ag 

91 

24 s 

Cadmium 

113 Cd((n,γ) 114 Cd 

20,000 

prompt 

Indium 

115 In(n) 116 n 

157 

54 min 

115 

In(n) 116m ln 

42 

14 s 

Samarium 

149 Sm(n,γ) 150 Sm 

41,000 

prompt 

I52 

Sm(n) 153 Sm 

210 

47 h 

Europium 

151 Eu(n) 152 Eu 

3,000 

9.2 h 

Gadolinium 

155 

Gd(n,γ) I56 Gd 

61,000 

prompt 

157 

Gd(n.γ) 158 Gd 

254,000 

prompt 

Dyprosium 

164 

Dy(n) 165 mDy 

2,200 

1.25 min 

164 

Dy(n) 165 Dy 

800 

140 min 

Gold 

197 Au(n) 198 Au 

99 

2.7 days 


PART 7. Semiconductors 

Certain semiconductor crystals, when exposed to ionizing radiation, become 

conductors and may be used as radiation detectors. Semiconductors are 

most often used for low level spectroscopic measurements of alpha particles, 

beta particles and gamma rays in laboratory settings and in X-ray diffraction 

equipment (Table 5). The most widely used semiconductor devices are 

diffused p-n junction, surface barrier and lithium drifted detectors. 

Semiconductor detectors have found their broadest application in the field of 

spectroscopy, although lithium drifted detectors are also being used for 

gamma ray detection. 


Detector 

The diffused p-n junction detector (Fig. 21a) gets its name from its 

manufacturing process. A slice of p type (electron depleted) silicon or 

germanium crystal, with a layer of n type (electron rich) impurity (usually 

phosphorus) deposited on the surface, is heated to form a p-n junction just 

below the surface. The phosphorus may also be painted onto the silicon and 

made to diffuse into it by applying heat. Because the n type material has an 

excess of electrons and the p type has an excess of holes (holes may be 

thought of as unit positive charges), the natural action of the combined 

materials tends to align the electrons on one side of the junction and the 

holes on the other. Thus a difference of potential is built up across the 

junction. By applying an external voltage to the crystal of such polarity as to 

oppose the natural movement of electrons and holes (reverse bias), the 

potential barrier across the junction is increased and a depletion region is 

produced. 


This depletion region is the sensitive part of the detector and is analogous to 

the gas volume in a gas ionization detector. Charged particles, on entering 

the depletion region, produce electron hole pairs analogous to the ion pairs 

produced in gas ionization chambers. Because an electric field exists in this 

region, the charge produced by the ionizing particle is collected, producing a 

pulse of current. The size of the pulse is proportional to the energy expended 

by the particle. 


Surface Barrier Detectors 

The operation of surface barrier and lithium drifted detectors is the same as 

for the p-n junction: a depletion region is produced, in which there exists an 

electric field. The means of producing the depletion region (as well as its 

dimension and location within the crystal) vary from one type of detector to 

another. The operation of a surface barrier detector (Fig. 21b) depends on the 

surface conditions of the silicon or germanium. At the surface of a piece of 

pure crystal, an electric field exists such that both holes and electrons are 

excluded from a thin region near the surface. For n type crystals, the field 

repels free electrons. If a metal is joined to the crystal, the free electrons are 

still repelled but a concentration of holes is produced directly under the 

surface. If a reverse bias is then applied, a depletion region is produced. 

Surface barrier detectors give better resolution for particle spectroscopy than 

p-n junctions but wider depletion regions are possible with the latter. (The 

wider the depletion region, the higher the energy of particles can be analyzed 

because a particle must expend all its energy in a depletion region.) 


Lithium Drifted Detectors 

The lithium drifted detector (Fig. 22) is produced by diffusing lithium into low 

resistivity p-type silicon or germanium. When heated under reverse bias, the 

lithium ions serve as an n type donor. These ions drift into the silicon or 

germanium in such a way that a wide layer of the p type material is 

compensated by the lithium, yielding an effective resistivity comparable to that 

of the intrinsic material. Wider depletion regions can be obtained with the 

lithium drift process than by any other means. Consequently, lithium drifted 

detectors are most useful in gamma spectroscopy work. Silicon detectors can 

be operated at room temperatures but exhibit low efficiency for gamma rays. 

Germanium detectors have higher gamma efficiencies but must be operated 

at liquid nitrogen temperatures. For these reasons, coupled with the small 

sensitive volumes obtainable to date, semiconductor detectors have not 

received widespread application in radiation survey instruments. 


TABLE 5. Radiation detector types. 


FIGURE 21. Cross sections: (a) diffused p-n junction detector; 

(b) surface barrier detector. 


FIGURE 21. Cross sections: (a) diffused p-n junction detector; 

(b) surface barrier detector. 


FIGURE 22. Cross section of lithium drifted detector. 


PART 8. Film Badges 

One of the most important uses of radiographic film as a means of measuring 

radiation is in film badges. Individuals who work with isotope radiation 

sources and X-ray machines are required by codes to wear badges indicating 

cumulative exposure to ionizing radiation. Film badges are discussed in this 

volume’s chapter on radiation safety and elsewhere 


Latent Image Formation 

Latent image formation is a very subtle change in the silver halide grain of film. 

The process may involve the absorption of only one or, at most, a few 

photons of radiation and this may affect only a few atoms out of some 10 9 or 

10 10 atoms in a typical photographic grain. Formation of the latent image, 

therefore, cannot be detected by direct physical or analytical chemical means. 

The process that made an exposed photographic grain capable of 

transformation into metallic silver (by the mild reducing action of a developer) 

involved a concentration of silver atoms at one or more discrete sites on the 

photographic grain. In industrial radiography, the image forming effects of X- 

rays and gamma rays, rather than those of light, are of primary interest. 

The agent that actually exposes a film grain (a silver bromide crystal in the 

emulsion) is not the X-ray photon itself but rather the electrons (photoelectric 

and compton) resulting from an absorption event. 

The most striking difference between X-ray and visible light exposures arises 

from the difference in the amounts of energy involved. 


The absorption of a single photon of light transfers a very small amount of 

energy to the crystal — only enough energy to free a single electron from a 

bromide (Br – ) ion. Several successive light photons are required to make a 

single grain developable (to produce within it, or on it, a stable latent image). 

The passage of an electron through a grain can transmit hundreds of times 

more energy than the absorption of a light photon. Even though this energy is 

used inefficiently the amount is enough to make the grain developable. In fact, 

a photoelectron or compton electron can have a fairly long path through a film 

emulsion and can render many grains developable. The number of grains 

exposed per photon interaction varies from one (for X-radiation of about 10 

keV) to 50 or more (for a 1 MeV photon). Because a grain is completely 

exposed by the passage of an energetic electron, all X-ray exposures are, as 

far as the individual grain is concerned, extremely short. The actual time that 

an electron is within a grain depends on the electron velocity, the grain 

dimensions and the squareness of the hit. A time on the order of 10 –13 s is 

representative. (In the case of light, the exposure time for a single grain is the 

interval between the arrival of the first photon and the arrival of the last 

photon required to produce a stable latent image.) 


Development 

Many materials discolor with exposure to light (some kinds of wood and 

human skin are examples) and could be used to record images. Most of these 

materials react to light exposure on a 1:1 basis — one photon of light alters 

one molecule or atom. In the silver halide system of radiography, however, a 

few atoms of photolytically deposited silver can, by development, be made to 

trigger the subsequent chemical deposition of some 10 9 or 10 10 additional 

silver atoms, resulting in an amplification factor on the order of 10 9 or greater. 

This amplification process can be uniform and reproducible enough for 

quantitative radiation measurements. Development is essentially a chemical 

reduction in which silver halide is converted to metallic silver. To retain the 

photographic image, however, the reaction must be limited largely to those 

grains that contain a latent image; that is, to those grains that have received 

more than a prescribed minimum radiation exposure. 


Compounds that can be used as photographic developing agents are those in 

which the reduction of silver halide to metallic silver is catalyzed (speeded up) 

by the presence of metallic silver in the latent image. Those compounds that 

reduce silver halide, in the absence of a catalytic effect by the latent image, 

are not suitable developing agents because they produce a uniform overall 

density on the processed film. 


Closing 

More information on the radiographic latent image, its formation and 

processing are available elsewhere. The correct use of film badges is 

especially important for safety in the conduct of radiographic testing programs 

and is discussed in this book’s chapter on radiation safety and elsewhere. 





Good Luck!

Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

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